1. Field of the Invention
The present invention concerns a novel road structure in which the bearing portion on top of the ground comprises three successive layers of bituminous material bonded with one another.
2. Background of the Invention
Road structures have long been constructed using natural or crushed aggregates, but initially without incorporating binders to enhance their cohesion. Nowadays the two techniques, i.e. incorporating binders or not, are used independently or in a combined fashion.
With a technology of the above kind employing structures made from untreated materials, the main design rule is the progressive nature of the moduli of rigidity. The materials with the lowest performance form a base layer which rests on the ground, with optional insertion of a forming layer; these are generally materials of low bearing capacity. They are followed by more elaborate materials and finally the wear layer which is the only layer constructed using a bituminous binder.
Table I below gives one example of a conventional structure made from untreated materials which illustrates the rule of progressive moduli.
TABLE I ______________________________________ ELASTIC LAYER THICKNESS MATERIAL MODULUS ______________________________________ Wear 6 cm Coated 5 400 MPa Base 20 cm 0/20 wetted 360 MPa reconstituted gravel Foundation 25 cm 0/31.5 120 MPa untreated gravel Ground -- Soil 20 MPa ______________________________________
Increasing traffic has led to the introduction of structures made from treated materials. These materials are sand or gravel treated either with bitumen or with cement or a similar hydraulic binder.
These structures are designed to operate in bending and are calculated accordingly.
In such cases, the progressive nature of the moduli between the foundation and base layers may still be present, but this is not the general rule, indeed the contrary is the norm.
In practice, a distinction is drawn between:
structures having base and foundation layers with the same rigidity (family F1 from table II below); the two layers are generally made of the same material or similar materials, and PA0 structures in which the base layer is significantly less rigid than the foundation layer (family F2 from table II below); in this case the function of the base layer is to prevent upward propagation of transverse shrinkage cracks that inevitably develop in layers of material treated with hydraulic binders. PA0 a) as a result of fatigue due to repeated flexing of the most heavily loaded treated layer, generally the foundation layer, and/or PA0 b) as a result of permanent deformation resulting from excessive vertical pressure on the untreated materials, in particular the soil. PA0 a base layer which rests on the ground with optional insertion of a forming layer of thickness H.sub.i such that: EQU 4 cm.ltoreq.H.sub.i .ltoreq.10 cm PA0 a median layer of thickness Hm such that: EQU 4 cm.ltoreq.H.sub.m .ltoreq.20 cm PA0 a top layer of thickness H.sub.s such that: EQU 4 cm.ltoreq.Hs.ltoreq.10 cm
From the structural point of view, the base and foundation layers are the thickest layers and are responsible for the longevity of the structure. The surface layer is responsible for the comfort and safety of the user.
TABLE II ______________________________________ Thick Structure Semi- bitumin- type rigid Concrete ous Mixed Inverse ______________________________________ Wear Coated Cement Coated Coated Coated layer materials concrete material material material Base MTHB Concrete Bitumin- Bitumin- WRG layer cement ous base ous base course course Founda- MTHB Lean Bitumin- MTHB MTHB tion concrete ous base layer course Family F1 F1 F1 F2 F2 ______________________________________ F1: Structures in which E.sub.CB .gtoreq. E.sub.CF F2: Structures in which E.sub.CB .ltoreq. E.sub.CF MTHB: Materials treated with hydraulic binders WRG: Wetted reconstituted gravel
Road structures can be damaged by two different processes:
Designing a road structure to prevent it breaking up therefore entails limiting these loads by increasing the thickness of the component layers of the structure so that they remain below permissible values that depend on the performance of the material and the traffic to which it will be subjected.